Why try to debug it?................ just look at their privacy policy:

We are commited to collaboration with authorities, especially prosecuting authorities and courts and can in this context be forced to pass on personal data.
Whilst I have never looked at the issue in depth I have always had a suspicion about these "one stop shop" encryption and delivery services. At the very least you must implicitly trust the provider? Pretty similar to anonymous proxy services IMO.

The most secure system I would imagine to be independently encrypting the message yourself before sending it. At least you wouldn't have to worry about backdoors.

Having said that, if the content is really sensitive, you shouldn't be using the internet anyway.